Update to helpbook-like documentation for WXT and Qt terminals

[Daniel J S. <dan...@ie...>]

I've updated a patch for adding the "wxhelp" in helpbook format to the 
WXT and Qt terminal windows:

https://sourceforge.net/p/gnuplot/patches/707/
https://sourceforge.net/p/gnuplot/patches/_discuss/thread/5dca9423/7c9e/5833/attachment/gnuplot-htmlhelp-2015jul11.patch

It is a nice way to access the documentation--fast, search-able, can 
freely navigate forward and back using scrollbar, plot examples.

Note that the patch does some reorganization because the documentation 
serves as a basis for both terminals, e.g., rather "helpbook" takes the 
place of "windows" in the source tree.  There are docs/wx and docs/qt 
subdirectories for the different bundles, and then when installed they 
end up in help/wx and help/qt subdirectories of the install location.

On the matter of html version of the documentation, the recent changes 
to the docs make file and usual mods occasionally finding their way into 
code have resulted in

make html
make install-html

not working so well.  Some issues:

1) Building html appears to put all sorts of PDF files in the docs 
subdirectory.  It might be better for that to be in some directory like 
docs/pdf.

2) Building gnuplot/html from outside the install directory seems to 
fail with a long, long stream of warnings about img3 (image003), img4 
(image004), etc. not being found.

3) Building gnuplot/html from inside the install directory works better 
but still has problems such as:

[snip]

Package: loading /usr/share/latex2html/styles/inputenc.perl
Warning: No implementation found for option: `utf8x' for `inputenc' package

Warning: No implementation found for option: `hyperindex' for `hyperref' 
package

Warning: No implementation found for option: `bookmarks' for `hyperref' 
package

Warning: No implementation found for option: `bookmarksnumbered_true' 
for `hyperref' package

[snip]

@@@@@@@@@@@@@@@@@@@@@@

*** no brace for \usebox , before:
\GpVersion{{
*** using "\GpVersion" as the argument instead; is this correct?  ***

@@@@@@@@@@@@@@@

*** no brace for \usebox , before:
\GpVersion
*** using "\GpVersion" as the argument instead; is this correct?  ***

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@


Dan